Recent improvements in microwave techniques have resulted in precise meas urements at the National Bureau of Standards, t he National Physical Laboratory , and elsewhere, which indicate t hat t he conventional constants K\= 79° K /mb and K 2'= 4,800° K in the expression for the refractivity of air , N = (n -1) 106= (K d T ) [p + K 2' (e / T ) ] should be revised . Various laboratories appear to have arrived at t his conclusion independently, wit.h the res ul t t hat t here are several different sets of constants in current use. In much of propagation work the absolute value of the refractive index of t he atmosphere is of small moment. However, in some work it is important, and it seems highly desiI able t o decide upon a particular set of constants.
Through consideration of t he various recent experiments a relation(e/ T)] is deri ved, where p is t he total pre s ure, in millibars, e is t he partial press ure of water vapor, in millibars, and T is absolute t emperature (O C + 273). This expre sion is considered to be good to 0.5 percent in N for frequencies up to 30,000 megacycles and normally encountered ranges of temperature, pI' ss ure , and h umidity.R ecent improvements in microwave techniques have resulted in measurements at the National Bureau of Standards [1]/ the National Physical Laboratory [2], and elsewhere [3 , 4, 5], which have indicated that the conventional constants in the expression for the r efractive index of air at radio fr equencies should be revised. Various laboratories appear to have arrived at this conclusion independently, with the result that there are several different sets of constants in current use [6, 7, , 9] . The sources of these recent changes, such as have b een run t o ear th, have been found to be ba ed on individual rather than collecLive resul ts. Almost all the proposed constants seem to represent a substantial improvement over the former values. The authors propose a set of constanLs derived from what is felt to b e the most r eliable 0f Lhe r ecent microwave and optical measurements of the r efractive index of .